To transform a digital image to a finished print of high quality requires that noise components, introduced because of the transformation of the image from an image medium to electrical signals be reduced and/or eliminated such that the noise does not become discernible to the human eye.
One of the methods receiving widespread use in the prior art is related to smoothing the differences between the values of the gray levels of pixels located in neighborhoods. A difficulty associated with this smoothing process is that it not only removes the noise components, hut it also blurs the edges. The edges exist when there is a transition region such as a region containing sharp detail, for example, grass which may define or outline a neighboring region which may he either smooth or sharply detailed.
A patent of interest for its teaching of noise reduction in a data processed image is U.S. Pat. No. 4,734,770 entitled "Image Data Processing Method and Device Therefor" by I. Matsuba. The method of that patent treats the first image data set, consisting of a plurality of pixels having values corresponding to their gray level some of which contain noise components, by transforming them into second image data to reduce the noise components appearing in the second image data. The method operates by selecting pixels to be treated and obtaining proposed values (candidate values) which are given to the object pixels based on a stated relationship. A probability function is used to determine whether a pixel in question should be raised to the candidate value or should be maintained at its present value. The probability formula includes the image energy component as one of its terms. A recognition is made for edges of patterns based on energy level comparisons, wherein a high energy level difference between a central and an adjoining pixel would have a high probability of defining an edge.
In the present invention a solution to the edge blurring, when performing the smoothing operation, approaches the problem by determining edge regions and by leaving those regions undisturbed or unoperated upon and by applying a smoothing function to the smooth regions so as to reduce noise in those areas. The method has two responsibilities, one to identify whether a pixel belongs to an edge or to a smooth region and, two, to compute a smoothing code value for the pixel if it belongs to a smooth region. Within this application, these two operations are called "segmentation" and "smoothing," respectively. Both of the aforementioned tasks are accomplished by a least squares fit of a plane to the image, in a neighborhood around the pixel of interest. If the goodness of the fit is small, then in that neighborhood the image is well approximated by a plane, and thus must be a smooth region. Further, the fit provides a smooth estimate for the code value at the pixel of interest which is the value of the fitted plane at the pixel of interest.